Voting in today's Michigan primary follows a week in which super PACs spent more than $2.7 million to influence the contest. And nearly all of that money was spent either in support of or in opposition to Rick Santorum.

Since Feb. 21, Mitt Romney's Restore Our Future super PAC spent $1.67 million opposing the former Pennsylvania senator, while Santorum's Red White and Blue Fund spent $773,700 in his support.

The filings provide some insight into who is perceived by Romney backers as the former governor's biggest threat. In the last week of January ...